After three weeks full of enough twists and turns to give one-day cricket a good name, it all comes down to this. The inaugural VB Series has had more permutations than a Rubik's cube, but now the maths are gloriously simple: Australia must beat South Africa at Perth tomorrow, and with sufficient élan to secure a bonus point, or they will have to leave their own party. If that happens, it will be the first time Australia have not qualified for the final of their annual triangular jamboree since 1996-97, and only the third time in the 22-year history of this tournament. The first time was in the inaugural series, in 1979-80, when Australia had a Wiener (Julian) and a Darling (Rick) opening the batting. This time they have an all-conquering squad comparable with any in Australian history, and the embarrassment of losing out to their New Zealand friends in particular would be considerable.

To gain a bonus point, Australia must score their runs at a rate at least 1.25 times South Africa's. In other words, if they bat second, they must reach their target within 40 overs. If they bat first and score, say, 250, they must restrict South Africa to 200 or less; if they get 280 the margin is 224 or less.

It's a tall order, even for this lot, and whether they manage it or not the the system for this tournament is likely to be reviewed. Unedifying though it was, the fault does not lie with New Zealand's go-slow - Steve Waugh did a very similar thing against West Indies in the last World Cup - but in the use of head-to-head results to split sides level on points.

New Zealand's 3-1 success ratio over Australia in this tournament means that run-rate does not come into play. If it did, Australia would need only to win - bonus point or no bonus point - as their run-rate is superior to New Zealand's, and was even before the Kiwis' Perth crawl.

A head-to-head system works fine in principle, and indeed in practice in football, but as soon as bonus points are introduced, it is open to abuse. These tournaments aren't big enough for both factors.

As for the teams, Australia are left with a straight choice between Ian Harvey and Darren Lehmann. If Harvey plays, a long-ish tail would begin at No. 7; if he doesn't, 10 overs will have to be fiddled from the batsmen - a potentially dangerous tactic given that runs are at a greater premium than usual. South Africa may make one change from the side that thumped New Zealand: Lance Klusener for Justin Kemp, who didn't bat and bowled only two overs in that match.

The implications of an Australian defeat, or even a narrow win, go beyond mere qualification. In one-day cricket at least, this could be the end of an era. Back in 1996-97, when they had to watch Pakistan and West Indies contest the finals, Australia quickly pensioned off their captain (Mark Taylor) and senior pro (Ian Healy).

For Taylor and Healy, read Waugh and Waugh: Steve and Mark have had poor series and the calls for their heads have become increasingly unsympathetic. The only difference is that 1996-97 came two years before the World Cup; there are only 12 months until South Africa and West Indies kick things off at Cape Town. Depth of talent or not, there is not much time for such major surgery.

The last time Australia were in such a do-or-die situation was at Headingley in the 1999 World Cup, when, like a slipshod adolescent in a slasher film, South Africa failed to nail their nemesis when they had the chance. They paid the price when they were emotionally hung, drawn and quartered in that agonising semi-final three days later, and cannot afford to make the same mistake again.

If Australia thump South Africa, the psychological high ground will be all theirs ahead of the best-of-three finals, and it shouldn't be forgotten that this series exists in the context of the forthcoming Test series between these sides, in which South Africa need only a draw to - officially at least - become the best side in the world.

Then there's the fact that South Africa have lost only one of their last 16 one-dayers to New Zealand. Chuck in the narrative of the last two triangulars involving these sides in Australia - South Africa qualify for finals, win the first match, then lose the last two to Australia amid allegations of choking - and it doesn't take a rocket scientist to see why South Africa will want to meet New Zealand in the final. It's a fair bet that, if Herschelle Gibbs takes a catch tomorrow, particularly off Steve Waugh, the ball will be going absolutely nowhere.

Probable teams

Australia 1 Mark Waugh, 2 Adam Gilchrist (wk), 3 Ricky Ponting, 4 Damien Martyn, 5 Steve Waugh (capt), 6 Michael Bevan, 7 Darren Lehmann, 8 Shane Warne, 9 Brett Lee, 10 Jason Gillespie, 11 Glenn McGrath.

South Africa 1 Herschelle Gibbs, 2 Gary Kirsten, 3 Jacques Kallis, 4 Neil McKenzie, 5 Jonty Rhodes, 6 Mark Boucher, 7 Lance Klusener, 8 Shaun Pollock (capt), 9 Nicky Boje, 10 Makhaya Ntini, 11 Allan Donald.

Rob Smyth is on the staff at Wisden.com.

Follow the game at Wisden.com with regular updates from Tanya Aldred in Australia

© Wisden CricInfo Ltd